1. Field of the Invention
This invention relates to a time delay switch of the kind capable of connecting a power consuming device to a power source for a predetermined time period and which utilizes a thermal element characterized by a temperature sensitive shape changing characteristic.
2. Description of the Prior Art
Thermal time switches have been known for a long time (German Pat. No. 705 383, German Disclosure Publication No. 25 44 758). They work generally in accordance with the principle of a bimetallic strip or of any expansion element which changes its shape after a certain time as a function of the temperature determined by the thermal and electrical characteristic data. In this way, the device can be operated to cut a circuit in and out.
Furthermore, the application of alloys with a shape memory is known for the interruption of electric circuits. The temperature control in which the memory effect takes place using a reacting spring has also been described (Swiss Pat. No. 616 270, European Patent 78200393.3).
Alloys with a shape memory are actually also known from numerous publications which will not be especially listed here. It is particularly a question of the alloy types Ni/Ti/Cu, Cu/Al/Ni and Cu/Zn/Al. The physical properties of such alloys with a shape memory are listed in the following table and are compared with those of bimetallic strips Fe/Ni.
______________________________________ Alloys Properties Ni/Ti/Cu.sub.10 Cu/Al/Ni ______________________________________ Density d 6.35 .times. 10.sup.3 kg/m.sup.3 7.2 .times. 10.sup.3 kg/m.sup.3 Specific heat Cp 2.98 .times. 10.sup.6 J/m.sup.3 K 3.32 .times. 10.sup.6 J/m.sup.3 K Latent heat .DELTA.H 110 .times. 10.sup.6 J/m.sup.3 60 .times. 10.sup.6 J/m.sup.3 Electric conductivity .sigma..sub.e 1.2 .times. 10.sup.6 S/m 9 .times. 10.sup.6 S/m Thermal conductivity .lambda. (20.degree. C.) 10 J/m K 75 J/m K Magnetic induction co-efficient &lt;1.002 .about.1 Max. work (2-way effect) 2 .times. 10.sup.6 J/m.sup.3 1.3 .times. 10.sup.6 J/m.sup.3 Switching temperature -200.degree. C. to -100.degree. C. to +100.degree. C. +200.degree. C. Overheating temperature +400.degree. C. +300.degree. C. Elasticity modulus 70 Gn/m.sup.2 75 GN/m.sup.2 Shearing modulus 15-25 GN/m.sup.2 35 GN/m.sup.2 ______________________________________ Alloys Properties Cu/Zn/Al Bimetal Fe--Ni ______________________________________ Density d 7.65 .times. 10.sup.3 kg/m.sup.3 8.1 .times. 10.sup.3 kg/m.sup.3 Specific heat Cp 3.07 .times. 10.sup.6 J/m.sup.3 K 4.06 .times. 10.sup.6 J/m.sup.3 K Latent heat .DELTA.H 30 .times. 10.sup.6 J/m.sup.3 -- Electric conductivity .sigma..sub.e 3 .times. 10.sup.6 S/m 1.25 .times. 10.sup.6 S/m Thermal conductivity .lambda. (20.degree. C.) 25 J/m K (?) 8 J/m K Magnetic induction co-efficient .about.1 Max. work (2-way effect) 1.0 .times. 10.sup.6 J/m.sup.3 0.02 .times. 10.sup.6 J/m.sup.3 (.DELTA.T = 100K) Switching temperature -100.degree. C. to -20.degree. C. to +90.degree. C. +300.degree. C. Overheating temperature +150.degree. C. (?) +500.degree. C. Elasticity modulus 60-70 GN/m.sup.2 Shearing modulus 35 GN/m.sup.2 ______________________________________
Customary time switches are distinguished by the fact that the active element (bimetallic strip or element expanding under the influence of the temperature) changes its shape only very little with a change in temperature and this change is, furthermore, effected in a continuous manner. This make the switches voluminous and expansive and the mechanisms determining the cut-in time can only be produced under great difficulties. An additional mechanism is necessary in order to ensure the clear cut-in and cut-out position of the switch owing to the absence of a temperature hysteresis of the active element. Therefore, there is a distinct requirement for the improvement and simplification of time delay switches in comparison with the traditional designs.